STRUCTURE AND STRATIGRAPHY OF THE NORTHERN LHASA TERRANE, TIBET

The northern Lhasa terrane of Tibet exposes primarily upper Paleozoic and Cretaceous sedimentary rock units. The upper Paleozoic strata consist of Carboniferous diamictite, quartzite, mudstone and shale, and Permian shale, mudstone and recrystallized limestone. The Cretaceous strata unconformably overlie the Paleozoic units and are ~4-km-thick. The lowest Cretaceous unit consists of volcanic rocks, which yielded U-Pb zircon ages of 131 ± 8 Ma, and overlying quartz- and volcaniclastic- rich sandstone and conglomerate of ~2 km thickness. Above this is a ~1-km-thick Aptian-Albian shallow marine limestone which shoals upward into primarily lacustrine marls and mudstone. E-W striking thrust relationships between these rock units are well exposed in cross sectional view along a N-S trending horst block, the result of late Cenozoic E-W extension in Tibet. The footwall of this horst consists of mostly Paleozoic strata repeated by north-directed thrust faults whereas the hanging wall of the horst consists of Cretaceous strata involved in mainly south-directed thrusting. A low-angle fault contact is locally exposed, placing younger Cretaceous strata over older Paleozoic strata. These relationships and the observation that the thrust faults disrupting the Cretaceous strata appear to merge into the low-angle fault suggest that the low-angle fault is a passive roof thrust separating two distinct structural levels of the thrust belt. The significance of these early findings are: (1) in contrast to previous suggestions, the northern Lhasa terrane was above sea level during much of Cretaceous time, and (2) interpretation of the dominant vergence and degree of lower crustal involvement in the thrust system is complicated by a zone of decoupling that exists between the uppermost and lower levels of the thrust system.